Optoelectronic devices include photovoltaic (PV) devices (solar cells), photodetectors, and like devices, as well as electroluminescent (EL) devices such as light-emitting diodes (LEDs) and laser diodes (LDs). A PV device generates electric power when light is incident upon its active layer. When sunlight is utilized as the source of incident electromagnetic radiation, the PV device may be referred to as a solar cell. In general, a PV device is based on a junction formed by a pair of two different types of semiconductors (e.g., an n-type and a p-type material, or an electron acceptor and an electron donor material). When a photon's energy is higher than the band gap value of the semiconductor, the photon can be absorbed in the semiconductor and the photon's energy excites a negative charge (electron) and a positive charge (hole). For the excited electron-hole pair to be successfully utilized in an external electrical circuit, the electron and the hole must first be separated before being collected at and extracted by respective opposing electrodes. This process is called charge separation and is required for the photovoltaic effect to occur. If the charges do not separate they can recombine and thus not contribute to the current generated by the PV device. A photodetector operates similarly to a PV device, but is configured to sense the incidence of light or measure the intensity, attenuation or transmission of incident light. Also, the operation of a photodetector entails the application of an external bias voltage whereas the operation of a PV device does not. Moreover, a photodetector is often intended to detect only a narrow range of wavelengths (e.g, an IR detector or UV detector), whereas a PV device is typically desired to be responsive to as wide a range of wavelengths as possible for maximum generation of electrical power.
Optoelectronic devices also include electroluminescent (EL) devices such as light-emitting diodes (LEDs) and laser diodes (LDs). In a general sense, EL devices operate in the reverse of PV devices. Electrons and holes are injected into the semiconductor region from the respective electrodes under the influence of an applied bias voltage. One of the semiconductor layers is selected for its light-emitting properties rather than light-absorbing properties. Radiative recombination of the injected electrons and holes causes the light emission in this layer. Many of the same types of materials employed in PV devices may likewise be employed in EL devices, although layer thicknesses and other parameters must be adapted to achieve the different goal of the EL device.
In PV and related optoelectronic devices, the efficiency with which optical energy is converted to electrical power is a key figure of merit. Another performance-related criterion is the open-circuit voltage Voc, the maximum possible voltage when the PV device is irradiated without being connected to any external load. Another performance-related criterion is the short-circuit current Jsc, the maximum possible current when the PV device is irradiated and electrically connected to a zero-resistance load. Another performance-related criterion is quantum efficiency, which includes both external quantum efficiency (EQE) and internal quantum efficiency (IQE). EQE corresponds to the percentage of total incident photons that are converted to electrical current, and IQE corresponds to the percentage of total absorbed photons that are converted to electrical current. Another performance-related criterion is the power conversion efficiency, which corresponds to the percentage of the incident optical power that is usable as electrical power.
In addition, charge carrier mobility within the constituent layers is a key material property that affects the performance of the device. Charge carrier mobility is a material property that describes the velocity of a charge carrier in the presence of an electric field. In PV devices a larger value of mobility means that charge carriers move more freely and can be extracted from the device more efficiently. This results in higher device performance as compared devices with lower charge carrier mobility. A related property is exciton diffusion length. This is a material property that describes the average distance that an exciton (an electron-hole pair) will travel before the charge carriers recombine. In a PV device where excitons play a significant role, a larger value means that there is a higher probability that photogenerated excitons will reach a charge separation region prior to recombination, and also leads to a higher device performance than a PV device with a lower exciton diffusion length.
While mobility and exciton diffusion are separate properties, their values are affected by similar material attributes. For example, defects, charge trapping sites, and grain boundaries all inhibit carrier transport and result in lower mobility as well as lower exciton diffusion length. While enhanced mobility is discussed throughout this document, it is understood that similar results are obtained for enhanced exciton diffusion length.
Conventionally, PV devices and other optoelectronic devices have utilized bulk and thin-film inorganic semiconductor materials to provide p-n junctions for separating electrons and holes in response to absorption of photons. In particular, electronic junctions are typically formed by various combinations of intrinsic, p-type doped and n-type doped silicon. The fabrication techniques for such inorganic semiconductors are well-known as they are derived from many years of experience and expertise in microelectronics. Nonetheless, these fabrication techniques are expensive. Successful crystal growth requires the minimization of defects and unwanted impurities, as well as the precise doping of intended impurities to achieve desired functions, in a high-vacuum, contamination-free deposition chamber under tightly controlled operating conditions. Group III-V materials such as gallium arsenide (GaAs) and AlxGayInzN (x+y+z=1, 0≦x≦1, 0≦y≦1, 0≦z≦1), as well as Si-inclusive compounds such as silicon carbide (SiC) and silicon-germanium (SiGe), have also been utilized but suffer from the same problems. Other inorganic materials such as amorphous silicon, polycrystalline silicon, cadmium telluride (CdTe), copper indium diselenide (CuInSe2 or CIS) and copper indium/gallium diselenide (CuInxGa(1-x)Se2 or CIGS) may be less expensive to fabricate than single crystal silicon, but are less efficient and still require expensive semiconductor-grade processing that has not yet reduced costs sufficiently to reach parity with traditional sources of electricity.
More recently, optoelectronic devices formed from organic materials (polymers and small molecules) are being investigated, but have enjoyed limited success. The active region in these devices is based on a heterojunction formed by an organic electron donor layer and an organic electron acceptor layer. A photon absorbed in the active region excites an exciton, an electron-hole pair in a bound state that can be transported as a quasi-particle. The photogenerated exciton becomes separated (dissociated or “ionized”) when it diffuses to the heterojunction interface. Similar to the case of inorganic PV devices, it is desirable to separate as many of the photogenerated excitons as possible and collect them at the respective electrodes before they recombine. It can therefore be advantageous to include layers in the device structure that help confine excitons to charge separation regions. These layers may also serve to help transport one type of charge carrier to one electrode, while blocking other charge carriers, thereby improving the efficiency of charge carrier extraction.
While many types of organic semiconductor layers can be fabricated at relatively low-cost, their power conversion efficiency has been lower than inorganic semiconductors due in part to short exciton diffusion lengths. Moreover, most organic semiconductor layers are ineffective for harvesting infrared (IR) photons, which is disadvantageous as IR radiation constitutes a significant portion of the radiation available for conversion to electricity or to other colors of light. As much as 50% or more of solar radiation are wavelengths longer than 700 nm. Moreover, organic materials are often prone to degradation by UV radiation or heat.
Even more recently, quantum dots (QDs), or nanocrystals, have been investigated for use in optoelectronic devices because various species exhibit IR sensitivity and their optoelectronic properties (e.g., band gaps) are tunable by controlling their size. Thus far, QDs have been employed in prototype optoelectronic devices mostly as individual layers to perform a specific function such as visible or IR emission, visible or IR absorption, or red-shifting. Moreover, optoelectronic devices incorporating QDs have typically exhibited low carrier mobility and short diffusion length.
A recent report by Dissanayake et al., “Measurement and validation of PbS nanocrystal energy levels,” Appl. Phys. Lett. 93, 043501 (2008), incorporated by reference herein in its entirety, described the use of a heterojunction between PbS nanocrystals (PbS-NCs) and C60 fullerenes to verify the band energy alignment of the PbS-NC layer. In this study, the PbS-NC layer was spun cast from toluene onto a buffer layer of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS), and the fullerene layer subsequently evaporated on top. This was followed by a layer of bathocuproine (BCP) and an aluminum electrode. The structure was tested in photovoltaic mode and provided a modest Jsc of ˜2 mA/cm2, a Voc of ˜250 mV, and therefore an overall power conversion efficiency of approximately 0.25%. No suggestions were made for methods or approaches to improve the performance of this device.
There is an ongoing need for optoelectronic devices with improved material properties and performance-related parameters such as more efficient charge separation, greater charge carrier mobility, higher open circuit voltages, longer diffusion lengths, higher power conversion efficiency, and sensitivity tunable to a desired range of electromagnetic spectra. There is also a need for lower cost, more reliable and more facile methods for fabricating such optoelectronic devices.